Thermal gradients in thermal analysis experiments: Criterions to prevent inaccuracies when determining sample temperature and kinetic parameters

Abstract

Thermal analysis methods are customarily used to analyze the evolution of solid-state transformations when samples are submitted to a controlled temperature program. Thermal analysis results are generally interpreted under the assumption that no temperature gradients take place inside the sample, so that, under proper calibration the sample temperature can be determined and controlled. Two phenomena may contribute to the formation of temperature gradients within the sample: heat transport through the sample and heat evolved during an exothermic or endothermic transformation. We will provide two analytical relationships that relate the sample mass with the temperature gradient within the sample and that will provide a straightforward criterion for checking the reliability of the sample temperature. We will show that, because of their very low thermal conductivity, significant thermal gradients may occur in those samples in the form of powders. Besides, in the case of exothermic reactions and for typical experimental conditions, the heat released by the reaction significantly affects the determination of the sample temperature. Finally, we analyze how sample overheating affects the observed reaction kinetics